A knowledge of the transmission characteristics of a signal path is required for the elimination of distortions at the receiving end. In many instances it is necessary to determine these characteristics both for the overall frequency range and for a number of narrower bands (e.g. voice channels) encompassed within that range. Such distortions include background noise, phase jitter and frequency-dependent amplitude and phase variations.
Conventional methods of determining the characteristics of transmission systems generally involve the evaluation, at the receiving end of a signal path, of a predetermined test signal fed to that path at the transmitting end. Thus, for example, the broad-band attenuation distortion can be measured by averaging the squared attenuation variations observed in different parts of the spectrum. This technique, however, disregards other signal-distorting parameters such as phase variations, nonlinearity and noise. A determination of attenuation and group delay as a function of frequency is usually carried out with the aid of a modulated sine wave, yet this requires a proper correlation of the modulation at the transmitting end with the evaluation at the receiving end which creates difficulties in the testing of long transmission lines. The frequency dependence of the group delay requires a time-consuming integration of the phase characteristics. Noise measurements are usually carried out in the absence of a test signal.